Try vinegar to treat discolored concrete deck

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Q. I have a 23-year-old concrete deck around my in-ground pool. After power washing it (just water) for 13 years, I noticed that the finish was mottled. Then I began using a 3-to-1 water-bleach mixture with a deck brush and elbow grease. The deck became clean, and so did the mottled sections. Then one spring I noticed real pockmarks in one section of the concrete, spalling of the mottled sections, like little craters. No winter chemicals ever touch the deck. The mottling doesn’t appear to have gotten worse, but the mottled sections do seem to absorb water differently — they dry faster, further highlighting the mottled effect. Is there a cause and the cure?

A. I think the mottling was caused by the type of mix that was used for the concrete, how it was installed, and how it cured. Also, 23 years is a fair time for concrete to serve, and I think the cure is to chop it up and pour a new deck. I also suggest you talk to a concrete contractor to see if he can figure it out. The spalling and pockmarks are from old age and weather. The spalling is due to the mottling spots absorbing water, which freezes, popping the spots right off.

Corrective measures, from the Portland Cement Association: Removing discoloration is more difficult than removing stains. Letting the concrete weather sometimes reduces the contrast in color but doesn’t always satisfy the owner. One solution: Apply pure vinegar and let it stand for 30 minutes before brooming off, using lots of water. A second application may be necessary. A contractor suggests treating dark discoloration by applying the flame from a hand-held blow torch directly to the darkened concrete surface. This reportedly lightens the dark spots but is time-consuming and probably not practical for large areas.

Q. I bought an Adirondack chair from the Christmas Tree Shop, which said it is made of cedar wood. I notice many pine-like knots in the wood. Could there be that many knots in cedar?

— ALICE KELLY, NEEDHAM

A. There are many grades of red cedar and white cedar, and, except for clear grades, both types can contain knots. Stain your chair with one coat of a semitransparent stain, which might make the knots fade. If you want to paint, you must shellac the knots with two coats of clear shellac, to prevent the knots from bleeding through. Only one coat of the semitransparent stain is needed.

Q. My husband and I bought a 1900 townhouse-style house in Malden. I have two questions: In July and August, we noticed a cat urine smell in a third-floor room. It seems to smell only when we have windows open on the third floor. We do know if the previous owner had a cat. The floors are original wide pine boards with gaps between the planks. How can we get rid of the odor?

Secondly, we’re trying to prioritize what to do next. After converting from oil to gas and replacing the water heater, we thought our next project would be to put a wood stove insert in our fireplace, thinking that this would reduce our gas bill. My husband thinks we should get the fieldstone and brick foundation up to par (mortaring the fieldstone and brick). The home inspector suggested we could fill in some of the mortar, but didn’t indicate that the fieldstone or brick had structural issues. Is this a priority, and could we do it ourselves?

— SUE, BY E-MAIL

A. For the cat urine smell, ventilation is slow, but will eventually eliminate the smell. Since the source (the cat) is long gone, there will be an end to the smell. Open windows in the room for cross-ventilation, and perhaps put an exhaust fan in a window. The odor is worse with open windows, which means the draft is pulling odors from the floor and gaps through the windows to the outdoors. Also, apply wintergreen-scented rubbing alcohol in the gaps between floorboards, which will ease the smell. Keep venting.

There is nothing to worry about the foundation. Sooner or later, it will need repointing, which is chipping out loose mortar and putting in new mortar. You can do it yourself: Buy Mortar Mix, a trowel, and a pointing tool. The latter is an elongated S-shaped steel bar that lets you press in new mortar. The secret is to compact that mortar heavily.

An alternative for the wood stove: Buy a pellet stove, which you can turn on and off with a flip of a switch. Bypass the chimney by extending the exhaust pipe through a wall to the outdoors. This pipe will exhaust fumes and bring in combustion air.

Peter Hotton is also in the g section on Thursdays. He is available 1-6 p.m. Tuesdays to answer questions. Call 617-929-2930. He (photton@globe.com) chats on Boston.com 2-3 p.m. Thursdays